Methods are known of separating into different amounts having different proteir contents, meals to be used as edible or as chemical-pharmaceutical products, said methods being based on air classification of the product, wherein the parts to be separated are placed in a series of cyclone separators which are serially traversed by the air entraining the product and are each provided, in the lower part thereof, with star-type rotary valves for continuous discharge of the amounts of precipitated product. The plants designed heretofore to carry this method into effect are of complicated construction, of difficult adjustment, of difficult adaptation and they are subject to so many inconveniences and drawbacks in their operation as to be unqualified for use on an industrial scale. In the known plants, the product is circulated through the various separators by a flow of pressurized air generated by an electric fan or similar device located upstream of the plant. These devices are, notoriously, sources of heat and, due to their location, they transfer said heat to the entire plant and, therefore, to the product so as to inevitably modify it as to nutritive capacity and/or other characteristics thereof. Meals discharged by the known plants, in fact, are characterized by a much darker shade than the meals originally fed to the plant.